As a modulation system for a transmitter in the field of mobile communications in a microwave band or mobile satellite communications, a narrowband linear modulation system such as four-phase shift keying (QPSK) or .pi./4 shift QPSK is adopted for a modulation system for a modulator in a transmitter in order to achieve effective utilization of frequency resources and reduction in transmission distortion.
Not only transmitters of the type mentioned are required to transmit modulated waves of a narrow band, but also a power amplifier thereof which amplifies a modulated wave is required to perform a strictly linear operation in order to assure an improved expansion of a spectrum of modulated waves. Accordingly, efforts for improvement of conventional transmitters have been directed to the elevation of the saturation level of the output of a power amplifier and the improvement of the linearity of the output.
Where a power amplifier having a non-linear phase characteristic is used, a transmitter which employs an automatic gain control (AGC) circuit and a phase modulation circuit as shown in FIG. 1 to suppress the expansion of a spectrum is conventionally known (Japanese Patent Laid-Open Application No. Heisei 4-287457, the title of the invention: "Phase Compensation System for Power Amplifier").
In the conventional transmitter just mentioned, as shown in FIG. 1, a modulated wave inputted by way of input terminal 1 and having a comparatively small amplitude distortion is supplied to AGC circuit 2, and simultaneously, an envelope of the modulated wave is detected by envelope detection circuit 6. The output modulated wave of AGC circuit 2 is phase modulated by phase modulation circuit 3 and then supplied to power amplifier 4, by which it is power amplified. The output signal of power amplifier 4 is outputted to output terminal 5 and simultaneously supplied to envelope detection circuit 8.
Differential amplifier 7 compares the envelope detection signal of the input modulated wave having a comparatively small amplitude distortion, which is inputted from envelope detection circuit 6, with the envelope detection signal of the modulated wave having an amplitude distortion generated upon amplification by power amplifier 4, which is inputted from envelope detection circuit 8, and controls the gain of AGC circuit 2 with a signal of difference between the envelope detection signals. Consequently, an amplitude distortion generated upon amplification of power amplifier 4 is suppressed.
Further, the envelope detection signal outputted from envelope detection circuit 6 is supplied to phase modulation circuit 3, and phase modulation circuit 3 controls power amplifier 4 so that it performs phase modulation of a characteristic opposite to the phase characteristic of power amplifier 4. Consequently, a phase distortion of the output signal generated by power amplifier 4 is suppressed, and degradation of the carrier power to noise power ratio (C/N) is reduced.
However, in the conventional transmitter which controls the expansion of a spectrum by raising the saturation level of the output of the power amplifier or by improving the linearity of the output, an increased cost is required for the power amplifier. Further, since the power dissipation increases, a large heat radiation mechanism is required, resulting in increase of the overall size of the entire transmitter.
Further, the conventional transmitter shown in FIG. 1 requires an expensive high frequency detection circuit for each of envelope detection circuit 6 and envelope detection circuit 8 provided on the input side and the output side of the transmitter, respectively, because the signal inputted to input terminal 1 is a phase modulated wave of a high frequency. Further, since the conventional transmitter requires two phase modulation circuits including another phase modulation circuit not shown, it is disadvantageous in that it requires a high cost. The conventional transmitter is disadvantageous also in that it cannot have variable output power.